Aqueous fluoropolymer dispersions have so far been used in manufacturing impregnated materials, such as roofing materials for membrane structures, by impregnating glass fibers, carbon fibers, Kevlar fibers and the like therewith and, further, in forming films by applying the same onto articles to be coated, followed by baking, in such fields of application as high-frequency printed circuit boards, transfer belts and packing materials.
Those aqueous fluoropolymer dispersions which are known in the art have the following problems caused, among others, by (1) the fact that increases in fluoropolymer concentration result in increases in viscosity, making the handling difficult and (2) the fact that when they undergo mechanical stresses on the occasion of transfer, film formation or like processing, fluoropolymer particles tend to form aggregates, resulting in deteriorations in quality of the aqueous dispersions and of films, impregnated articles and the like obtained by using the same (the so-called mechanical stability problem). These problems are significant particularly at low fluorinated surfactant content levels.
As high-concentration aqueous fluoropolymer dispersions low in viscosity, there have been proposed fluoropolymer dispersions containing a fluorinated surfactant having a molecular weight of lower than 1000 at a solid concentration level lower than 0.025% by weight and containing a nonfluorinated nonionic surfactant and a surfactant selected from the group consisting of fluorinated anionic surfactants having a molecular weight of not lower than 1000, nonfluorinated anionic surfactants, and mixtures thereof (cf. e.g. Patent Document 1) and fluoropolymer dispersions which contain a nonfluorinated nonionic surfactant or a mixture of a nonfluorinated nonionic surfactant and a nonfluorinated anionic surfactant as so selected as to provide a specific viscosity characteristic (VTT) and which contain a fluorinated surfactant having a molecular weight of lower than 1000 at a solid concentration level not higher than 0.05% by weight (cf. e.g. Patent Document 2).
However, any composition to be added to such fluoropolymer dispersions for maintaining their mechanical stability is not disclosed in those documents.
For the purpose of achieving improvements in mechanical stability, a composition resulting from incorporation, in an aqueous PTFE dispersion obtained by emulsion polymerization, of a specific nonionic surfactant and polyethylene oxide having a specific average molecular weight or a water-soluble polyurethane-based associative thickener, each in an specific amount has been proposed (cf. e.g. Patent Document 3). However, this technology is not intended for reducing the content of the fluorinated surfactant used in emulsion polymerization.    Patent Document 1: United States Patent Application Publication 2004/186219    Patent Document 2: United States Patent Application Publication 2004/171736 ([0016])    Patent Document 3: Japanese Kokai Publication 2000-198899 (claim 1, [0019])